The present invention relates to compensating circuits for transformers and is directed more particularly to circuits for reducing the effect of stray capacitance on signal transmission through transformers.
Because of their usefulness in providing isolation, impedance matching and voltage and current transformations, transformers have become widely used in circuits such as voice-frequency amplifiers. In a voice-frequency amplifier circuit such as, for example, that described in the U.S. Pat. of Charles W. Chambers, Jr., No. 3,706,862, entitled "Amplifier Circuit For Transmission Lines," transformers play a useful role in coupling the amplifier circuitry in series with and across a telephone line.
One serious problem with utilizing transformers for coupling voice-frequency signals is that the windings thereof exhibit a stray, distributed capacitance between the turns of the windings thereof. This stray capacitance causes a portion of the driving signal current to traverse the terminals of the primary winding without inducing corresponding currents in the secondary windings, particularly at the high end of the transmission band. The presence of this stray capacitance has been found to worsen the impedances of transmission lines which include such transformers.
Prior to the present invention it has been the practice to minimize the effect of stray capacitance by utilizing transformers having winding configurations which minimize the distributed capacitance between the turns. The improvement which could be effected in this manner was, however, quite limited and not subject to change once the transformer was made. In addition, such improvements could not be made in existing transformers.
In accordance with the present invention, there is provided capacitance compensating circuitry which improves the high frequency response of transformers having ordinary core materials and ordinary winding configurations. More specifically, the compensating circuitry of the invention is adapted to substantially cancel the stray capacitance of transformers and thereby extend the band of frequencies which may be coupled through the transformer without significant frequency dependent effects. In addition, the circuit of the invention is adapted to allow the desired capacitance cancellation to occur over a controllable band of frequencies extending far beyond that normally provided by ordinary transformers and to suppress that capacitance cancellation for signal frequencies outside of the desired band.